Electrostatic coating apparatus



April 10, 1951 c. KAHAN 2,548,477

ELECTROSTATIC COATING APPARATUS Filed July 23, 1945 2 Sheets-Sheet 1 IN VEN TOR. Gym? E5 KHHHN,

April 10, 1951 c. KAHAN 2,548,477

ELECTROSTATIC COATING APPARATUS Filed July 23, 1945 2 Sheets-Sheet 2 Patented Apr. 10, 1951 OFFICE- ELECTROSTATIC COATING APPARATUS Charles Kahan, Los Angeles, Calif. Application July 23, 1945, Serial No. 606,539

6 Claims.

This invention relates to apparatus for applying a coating to a series of articles, and more particularly to apparatus embodying an electrode past which the articles are successively moved and between which and the articles an electrostatic field is maintained. Letters Patent No.

2,359,476, granted October 3, 1944, on the application of Charles K. Gravely illustrates one form of such apparatus in which the electrostatic field is employed to remove excess liquid coating material. a

In the application of paint or other liquid coat'-' ing materials by the dip process, the excess material which remains at the drain-off points after draining has substantially ceased has heretofore been a problem which has seriously limited widespread application of this very efiicient coating method. As a result of the use and application of the method and apparatus disclosed in the above-mentioned patent, which method is now generally termed electrostatic detearing, the removal of excess coating material is effected satisfactorily. This removal of excess material is obtained by so arranging the conveying apparatus which conveys the articles through the coating bath that the articles, after having been coated, move at the correct time past and in spaced relation to a detearing electrode maintained at a high electrical potential relative to the articles. This high potential establishes about the articles an electrostatic field which causes excess coating material to leave the article and move toward the detearing electrode.

In electrostatic detearing it is highly important that the apparatus be so arranged that the correct distance between the grounded articles and the charged electrode be maintained. The articles must be securely attached to the conveyor and spaced from the detearing electrode so that there will be no possibility of a spark occurring between the articles and the detearing electrode. If such a spark occurs, the electrostatic field to all articles which are within the influence of the grid at that time is reduced below the value required for efiective detearing and the apparatus isthus rendered temporarily inoperative. ;The

prevention of sparking is especially desirable when the coating material is of an inflammable nature.

However, although all feasible precautions may be taken to insure that the distance between the electrode and articles on the conveyor is always greater than the spark-over distance, there is no assurance against articlesfalling from the conveyor on to a detearing electrode disposed beneath 2 the articles. a capacity charge, and a'spark would occur as it approached the grid. Moreover, the presence of a fallen article on the detearing electrode will, in effect, raise the surface of the electrode closer to the path of other articles on the conveyor than the planned spacing therebetween and further sparking may occur as following articles pass-overthe fallen one. In spite of all reasonable care, other conditions may occur in which one or more of the articles being deteared will approach with-- in sparking distance of the conveyor.

Heretofore, it has been. deemed necessary to have the space between the electrode and the articles being deteared free of any interposed material so that the detearing field could be established without hindrance or interference. Under these conditions the detearing electrode naturally collected a considerable quantity of the coating material which deteared from the articles. This material not only resulted in unsightly and unsafe conditions but if enough of it collected on the detearing electrode it might redetear either on to the coated articles to mar their surface or onto other objects where its presence would be objectionable.

An important object of this invention is to provide electrostatic coating apparatus with means for avoiding any possibility of a spark and thereby to avoid the inoperativeness and danger which sparking would cause.

Another purpose of this invention is to screen objects from approaching too closely to the oppositely charged electrode.

A further object is to limit the approach of articles to the electrode without materially interfering with the desired action of the electrostatic field.

A further object is to prevent the accumulation of coating material on a detearing electrode and thereby-to avoid the troublesome redetearing of this material from theelectrode on to other objects.

The full nature of the invention will be understood from the accompanying drawings and the following description and claims:

Fig. 1 is a diagrammatical illustration showing the relation of the dip tank and the detearing electrode with respect to the articles conveyed therethrough; Fig. 2 is a perspective view of one of the preferred embodiments of the invention with portions broken away; and Fig. 3 is the same as Fig. 2-, illustrating a different formof the invention.-

J In the drawings there is illustrated a dip coat-.

Such a falling article would retain ing apparatus for articles, wherein there is provided a dip tank I containing a bath of coating material. Mounted over the dip tank and extending away therefrom there is a conveyor H of the usual character having a series of hangers 12 for supporting the articles I 3 to be coated. Spaced from the dip tank there is provided a frame support M of electrically insulating material upon which an electrically conducting detearing electrode I5 is mounted. The support [4 is suspended by adjustable electrically insulating rods [6 from an overhead support 11. The frame with its grid 15 is suspended in position for the conveyor I l to:carry thelarticlesr l3 thereover after having been coated with material from the dip tank l8.

As indicated in Fig. l, the conveyor and. the

Voltage source, a strong electrostatic fieldcwill be created.

The creation ofsuch a field and the possibilitiesiof sparkingttherein, in a region where the.

atmosphere may contain a quantity of inflammable solvent vapors from the coating material, introduces a potential hazard in theuse oi such equipment. Such sparks may be caused by an articledropping from the hanger onto the electrode, by an oversized article passing nearer to the electrode than the spark-over distance, orby any other situation which will shorten the distance between the electrode and the article. An additional undesirable condition exists insofar as excess material removedfrom the articles by the action of the field will accumulate and build up on the grid. As' the deteared coating material builds up on the electrode, it hasthe same eirect as though the charged electrode were moved closer to the articles, particularly when the coating material is a good conductor. Further accumulated coating material onthe electrode will tend to redetear back on tothe articles or on to other objects;

For overcoming these problems, an imperforate insulating screen 22' composed of material of relatively poor electrical conductivity maybe mounted over the detearing electrode and un-- der the path of travel of the articles. As shown, this screen of insulating material is supported upon the electrode support l4 above the detearing electrode ata sufiicient-di'stance so as-to preclude the possibility of a spark from an arti of approximately four" ing material is deteared to the surface of the screen closest to the articles and is therefore not allowed to come within sparking distance of the detearing electrode. Furthermore the surface of the screen to which the excess material is deteared is electrically floating, so to speak, being insulated from either terminal of the electrostatic field. In the arrangements shown herein, air or other gaseous matter, as well as the insulated supportswhich are installed in this structure, may constitute this insulation. From such a floating surface the redetearing ofaccumulated coating material will be greatly minimized.

Whereas in Fig. 2 the insulating screen is shown in: the form of an imperforate sheet of material and,. therefore, is capable of protecting the electrode from accumulation of coating material; Fig. 3 is illustrative of another form of insulating screen which; is equally effective in most instances from the standpoint :of protection against sparking and'fire'hazards, and may be amore desirable construction where materialaccumula-tion on a detearing electrode is of minor importance or where a substantially unobstructed.

electrostatic field is desired. The form of insulating; screen illustrated-in Fig. 3 comprises: a:

series of spaced insulating rods I22 mounted .in

the upper frame of thesupport M and extending.

transversely of the electrode. Said rods areso spaced that if any article l3 should fal1, -it will be supported on the-rods and not'pass-therebetween.

guard screen I22 in the modifications'shown in Fig. Brnust be grea-ter than the maximum-sparkover' distance from the detearing electrode to thegrounded articles, this distance in'the modifica=- tion shown in Fig- 2 may be varied depending upon the thick-ness and dielectric strength of the. material fromrwhiclr the'screen 22 is formed.

In the modifications of the invention shown in Fig. 2: the detearing electrode is protected: not

only from above by the screen 2zrbut also from theends and sides by walls 25' and from beneath Such complete enclosure-1 of the grid'maybeemployed: when" it is desired: to prevent any'object or'person'from approaching within sparking distance of the gridirome any:

by a bottom'shield 26.-

direction, and may be eliminated. if' desired. However, if the walls-25 and 26 an'd lower screen are eliminated, the screen above'the electrode:

desirably" extendshorizontally beyond the end of the electrode far'enough to intercept anyarticle'" projecting downwardly too far' from the con veyor I l and thereby to prevent such an article the end of the electrode. is shown in Fig. 3.

Although the insulating screen has been shown' hereinas applie'd'to a detearing electrode over which the articles" pass, such a protective insulat--- ing screen is equally effective for protection. against the same-'hazards 'wh'en interposed between the path of the articles and a detearing:

electrode or electrodes-mounte'd at the side of: the artiblesyor when in any'other arrangement thereis danger that the articles will approach within sparking distance-of an oppositely charged: electrode'.

The material of which the screen above the detearing electrode is formed isreIati'i/ely immaterial so long as it is not a good electrical con ductor; such asa metal, If the' scree'n is imperforate, like the screen 22 of Fig. 2, and of high dielectric Strength, it may be mounted with its upper surface somewhat closer to the electrode than could a foraminous screen or one of relatively little dielectric strength; for, if the screen is imperforate, its dielectric strength will augment that of the air-gap beneath it in opposing sparking between the electrode and an article resting on the upper surface of the screen.

Because of the danger of sparking between an electrode and an article passing it, it has heretofore been the practice to arrange the electrode so that the minimum distance between it and the normal path of the article moved past it would be about twice the spark-over distance at the potential difference maintained. between the articles and the electrode. Thus a margin of safety was provided, although at the expense of a reduction in the strength, and hence the efiectiveness, of the electrostatic field. By providing an insulation screen in accordance with this invention, the articles coated are positively prevented from approaching within spark-over distance of the electrode, and the distance between the electrode and the normal path of the articles may be reduced below the minimum heretofore regarded as safe, thus increasing both the strength and effectiveness of the field.

I claim as my invention:

1. In apparatus for coating a series of articles with a liquid coating, a conveyor for moving said articles successively over a predetermined path, an electrode spaced laterally from said path, means for maintaining a high electrical potential difference between said electrode and said articles as the latter move past the electrode, and an insulating screen disposed in such spaced relation between said electrode and the articles moving past it as to limit the approach of the articles to the electrode to a minimum distance greater than that across which a spark can jump in air from the articles to the electrode at the potential difference maintained between them.

2. The invention set forth in claim 1 with the addition that said conveyor is arranged to move said articles past said electrode after they have been coated and while coating on the articles is still liquid, whereby the electrostatic field existing between said articles and electrode will cause excessive coating material to leave the articles, said electrode being located below the articles moving past it.

3. The invention set forth in claim 1 with the addition that said conveyor is arranged to move said articles past said electrode after they have been coated and while coating on the articles is still liquid, whereby the electrostatic field existing between said articles and electrode will cause excessive coating material to leave the articles. said screen being provided with a plurality of openings each of which is small enough to prevent the passage of one of said articles through it.

4. The invention set forth in claim 1 with the addition that said conveyor is arranged to move said articles past said electrode after they have been coated and while coating on the articles is still liquid, whereby the electrostatic field existing between said articles and electrode will cause excessive coating material to leave the articles, said screen being imperforate.

5. In apparatus for applying a liquid coating to the surface of an article, a support for the article, an electrode spaced from the article on said support, said electrode and support being arranged for relative movement, means for maintaining a high electrical potential difference between the electrode and the article on the support, and an insulating screen disposed in spaced relation between. the electrode and the article to limit the distance therebetween to a minimum distance greater than that across which a spark can jump from the article to the electrode at the potential difierence maintained between them.

6. In an article-coating apparatus, an electrode, a conveyor for moving the article past and in spaced relation to the electrode, means associated with the conveyor for applying an excess of liquid coating material to the article before it is moved past the electrode, means for maintaining an electrical potential difference between the electrode and the article passing it capable of causing excess coating material to leave the article and move toward the electrode under the influence of electrostatic forces, and a means for preventing coating material leaving the article from engaging the electrode and thereby acquiring a charge such as would tend to cause its return to the article, said last named means comprising a screen of insulating material interposed between the electrode and the article for intercepting such excess coating material before it reaches said electrode.

CHARLES KAHAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,033,991 Melton Mar. 17, 1936 2,187,624 Melton et a1. Jan. 16, 1940 2,334,648 Ransburg Nov. 16, 1943 2,359,476 Gravley Oct. 3, 1944 2,371,605 Carlton et al Mar. 20, 1945 2,417,798 Ransburg et a1. Mar. 18, 1947 

